Variations in the rate of net acid excretion by the mammalian kidney contribute importantly to the homeostatic regulation of systemic arterial pH. The thick ascending limb of Henle's loop has been recognized only recently as an important site involved in urinary acidification. This nephron segment contributes directly to the renal excretion of net acid by reabsorbing bicarbonate. In addition, the thick ascending limb reabsorbs ammonia, a process which may facilitate transfer of buffer to the collecting ducts and promote urinary acid excretion. No direct information is available regarding the regulation of these transport processes. Therefore, the goal of this proposal is to examine factors that may contribute to the regulation of ammonia and bicarbonate transport in the cortical and medullary thick ascending limb of the rat. The isolated, perfused tubule technique will be used to investigate four specific issues: I.) The potassium-dependence of thick ascending limb ammonia and bicarbonate transport, II.) The role of chloride in thick ascending limb bicarbonate and ammonia transport, III.) The effects of changes in acid-base balance in vivo on thick ascending limb bicarbonate and ammonia transport in vitro, and IV.) The role of hormones in regulation of thick ascending limb acid-base transport. These specific factors have been chosen because each is known to have important impact on renal acid excretion and because changes in potassium balance, chloride balance and systemic hormone levels are commonly altered during clinical acid-base disorders. These experiments will be the first to examine directly regulation of acid-base transport in the mammalian thick ascending limb. The results of these studies should provide important new information regarding the contribution of the thick ascending limb to changes in renal acid excretion associated with a variety of pathological conditions.